It is the moment when ordinary fans transform into body-language experts. The final day of the season and the so-called lap of appreciation, which, given Arsenal's plight this season, would be better described as a lap of depreciation.

Who's waving a little too energetically as they hold their infant child by their shoulder? Who looks suspiciously misty-eyed? This kind of scrutiny will be nothing new for Alexis Sanchez, of course: Studying his body language has become a cottage industry in its own right this season. Column inches have been dedicated to discarded gloves, arms thrown up in the air, angry gesticulations and, against Swansea back in January, a jacket placed over the head.

On Sunday, when Arsenal's Premier League season ends against Everton and, most likely, their campaign to qualify for the Champions League finally collapses, the nature of the analysis will change. No longer will it be a debate over whether Sanchez is a "born winner" or just a sulk; whether his actions are carefully calibrated to send a message amid difficult contract talks or instinctive moments which betray his frustration with the limitations of his teammates. The only question will be whether, after all that has passed, Sanchez will stay at the club.

If Arsenal do fail to overtake Liverpool or, even more unlikely a proposition, Manchester City, on the final day, you would imagine Sanchez's mind will be made up for him. It has probably been decided already, and an exit has seemed all but certain for some months now. But a player of his extreme talent would not contemplate missing out on Champions League football next season. That would surely be the end of any doubt.

Everything points to Arsenal losing their best player. Again. The signings of Mesut Ozil in 2013 and Sanchez a year later were supposed to herald a new era of team-building, the Emirates Stadium transition finally coming to fruition. And yet this summer could represent a return to 2012, when Arsenal last possessed a player on a similar level to Sanchez, only to see him depart to a league rival.

Robin van Persie scored 30 league goals in his final campaign, and with 23 this season, Sanchez will not trouble that mark. But in terms of his overall influence on the team, his ability to settle matches at will and his mix of penalty-box predation and top-class creation, it has been a similar effort. Losing Van Persie was traumatic, doubly so as he joined Manchester United, and the prospect of Sanchez moving to Chelsea or Manchester City this summer should invoke similar dread. This season he has become truly world class, excelling at multiple positions and improving his passing to no end. There is no obvious replacement.

With Champions League football unlikely, Sunday figures to be Alexis Sanchez's last home game at the Emirates.

Finishing below Tottenham for the first time since 1995 and, if confirmed at the weekend, slipping out of the top-four for the first time under Arsene Wenger would both be significant indicators of Arsenal's decline as a force. Losing their best player amid a contract standoff when the club are now supposed to be equipped to operate at the top of the game would arguably be equally as telling.

For the supporters, it is more simple. Sunday could be the last time they see Sanchez at Emirates Stadium -- in an Arsenal shirt at least. Despite the rumblings around tensions in the dressing room, and the eruption of controversy around his exclusion from the starting XI at Liverpool, he has comfortably been the best thing about this season and consistently the most exciting player to watch. It is an uncomfortable realisation that he may have only 90 minutes left in his Emirates Stadium career.

The FA Cup final is the true coda to the season, but when Sanchez strolls around the stadium with the other Arsenal players following the final whistle against Everton, it may effectively be his goodbye moment, and a painful one at that.

It was not until this season that Arsenal found a truly worthy successor to Van Persie, and now they may have to find another. It will make Sunday a difficult occasion, whatever happens on the pitch and in the league table.

Tom is one of ESPN FC's Arsenal bloggers. You can follow him on Twitter @tomEurosport